The present invention relates to a phase-shift photo mask blank consisting of a multilayered phase-shift film and a phase-shift photo mask as well as a method for the fabrication of a semiconductor device using the mask. More specifically, the present invention pertains to an attenuation type (half tone) phase-shift photo mask and phase-shift photo mask blank for use in the production of the photo mask as well as a method for the fabrication of a semiconductor device using the mask.
There have conventionally been proposed, as attenuation type phase-shift photo masks, those comprising single-layered films (see, for instance, Japanese Un-Examined Patent Publication No. Hei 7-140635) and those comprising double-layered films (see, for instance, Japanese Un-Examined Patent Publication No. Hei 8-74031). The film structure of an attenuation type phase-shift photo mask comprising a single-layered film is shown in FIG. 1 and that of the photo mask comprising a double-layered film is shown in FIG. 2.
Patterns of semiconductor integrated circuits have increasingly been finer as the recent development of the technique for the fabrication of a semiconductor integrated circuit and this in turn leads to the reduction of the wavelength of light rays used for exposure. For this reason, the following characteristic properties have been required for the attenuation type phase-shift photo mask, which attenuates the intensity of exposed light rays having a desired wavelength:
(1) The phase difference (PS) should satisfy the following relation: PS=175 to 180 degrees;
(2) The transmittance (Texp) at the wavelength (xcexexp) of the exposed light rays should satisfy the following relation: Texp=2 to 30%;
(3) The transmittance (Tinsp) at the inspection wavelength (xcexinsp) should satisfy the following relation: Tinsp less than about 40 to 50% (for instance, xcexinsp=365 nm when xcexexp=193 nm);
(4) The reflectance (Rexp) at the exposure wavelength should preferably satisfy the following relation: Rexp less than about 20%; and
(5) The film thickness d is preferably thin.
If the exposure wavelength (xcexexp) is reduced (for instance, xcexexp=193 nm for the exposure using an ArF excimer laser) in the case of the foregoing conventional phase-shift film, the transmittance (Texp) at the exposure wavelength (xcexexp) is lower than the foregoing required transmittance. For this reason, if the transmittance (Texp) is increased in order to ensure the required transmittance at the exposure wavelength, the transmittance at the defect-inspection wavelength becomes extremely high. For instance, in the case of the double-layered film as shown in FIG. 2 and in the case of the foregoing conventional technique, the transmittance at the exposure wavelength is reduced since the refractive index of the upper film (n1xe2x88x92ik1), which comes in contact with the ambient environment such as air (refractive index: no) or other gases, is greater than that of the lower film (n2xe2x88x92ik2). As a result, any satisfactory transmittance at the wavelength of, for instance, an ArF excimer laser cannot be obtained. As shown in Tables 1 and 2, the transmittance of a phase-shift film tends to become higher as the wavelength becomes longer. On the other hand, if the transmittance at the exposure wavelength is increased, the transmittance at the defect-inspection wavelength (for instance, xcexinsp=365 nm) for a conventional phase-shift photo mask becomes extremely high, making defect-inspection impossible. As previously discussed, the foregoing conventional techniques are not compatible with the recent development of the techniques for the fabrication of semiconductor integrated circuits. For example, among the basic structures F11, F13 in which the number of layer is one in Table 1, F11, has a phase difference of about 180xc2x0 at the exposure wavelength of 193 nm with the transmittance of 10.57%, which meets the requirements (1) and (2) described on page 1, line 26 to page 2, line 1. However, if the inspection wavelength is limited to 365 nm, the transmittance of the phase-shift film is 86.52%, which does not meet the requirements under (3) at page 2, lines 2-4. However, if the defect-inspection wavelength is 248 nm, the transmittance is 27.82% as shown for a single layer film F11 in Table 1. Thus, when a defect inspection device is used at an inspection wavelength of 365 nm, there is a resultant large scale increase for the transmittance. This allows one to perform a further inspection with a second defect inspection device using a shorter defect-inspection wavelength.
In this connection, the wavelength used for inspection is likely to be reduced owing to the efforts of manufacturers of defect-inspection devices. Therefore, fewer problems should arise if the transmittance increases to an extent beyond an acceptable value for the defect-inspection wavelength (365 nm) by shortening the defect-inspection wavelength. Thus, it is likely that a new defect inspection device, which can use a considerably shorter wavelength than 365 nm, is achievable in the near future.
In summary, it is probable that the next generation of defect-inspection devices will allow for a slight increase in the transmittance at the defect-inspection wavelength of 365 nm. However, it seems less likely that a great increase in the transmittance can be achieved without being problematic for defect inspection.
Accordingly, it is an object of the present invention to provide a phase-shift photo mask, which can solve the foregoing problems associated with the conventional techniques, which can ensure a transmittance sufficient for shorter exposure wavelengths and therefore, permits the use of such a shorter exposure wavelength, and which has an appropriate transmittance for the defect-inspection wavelengths and thus permits the satisfactory inspection.
Another object of the present invention is to provide a phase-shift photo mask blank for use in making such a photo mask.
A further object of the present invention is to provide a method for fabricating a semiconductor device, which makes use of the photo mask.
The inventors of the present invention have conducted various studies to solve the foregoing problems associated with the conventional techniques, have found that the foregoing problems can effectively be solved by taking notice of the refractive indexes of films or layers and optimizing the structure of a multilayered phase-shift film and thus have completed the present invention.
According to an aspect of the present invention, there are provided phase-shift photo mask blanks, which comprises a half tone phase-shift film, wherein the half tone phase-shift film comprises two layers and the upper layer of the film has a refractive index smaller than that of the lower layer of the film. The photo mask blank permits the production of a phase-shift photo mask whose transmittance at the exposure wavelength is high and whose reflectance is low.
According to another aspect of the present invention, there are provided phase-shift photo mask blanks, which comprise a half tone phase-shift film, wherein the half tone phase-shift film comprises three layers and the refractive index of the intermediate layer of the film is smaller than those of the upper and lower layers of the film. Thus, the resulting phase-shift photo mask has a low transmittance at the defect-inspection wavelength and this in turn permits the inspection. In the case of the phase-shift film comprising three layers, the film may have, for instance, the following basic structure: air or other gases/n1, k1, d1/n2, k2, d2/n3, k3, d3/transparent substrate (wherein n1, n2 and n3 are refractive indexes of the upper, intermediate and lower layers, respectively; k1, k2 and k3 are extinction coefficients of the upper, intermediate and lower layers, respectively; and d1, d2 and d3 are thicknesses of the upper, intermediate and lower layers, respectively).
Moreover, if the half tone phase-shift film according to the present invention comprises three layers, it is also possible to reduce the refractive index of the upper layer of the film to a level smaller than that of the intermediate layer thereof. In this case, the photo mask blank also permits the production of a phase-shift photo mask whose transmittance at the exposure wavelength is high and whose reflectance is low.
The half tone phase-shift photo mask blank according to the present invention may further comprise a half tone phase-shift film comprising at least 4 layers, wherein the refractive index of the upper most layer of the film is smaller than that of the layer immediately below the upper most layer. Thus, the photo mask blank permits the production of a phase-shift photo mask whose transmittance at the exposure wavelength is high.
The half tone phase-shift film is an MoSiON type film. In addition, the phase-shift film may likewise be an MoSiN type or MoSiO type one.
The phase-shift photo mask according to the present invention comprises the foregoing phase-shift photo mask blank on which a pattern to be transferred to a wafer substrate is formed according to the usual method. A semiconductor device having a fine pattern can be fabricated by exposing a semiconductor wafer to light rays through the phase-shift photo mask.
Accordingly, the present invention further relates to a method for fabricating a semiconductor device having a fine pattern, which comprises the step of exposing a wafer substrate to light rays through a phase-shift photo mask produced from the foregoing phase-shift photo mask blank.
According to a further aspect of the present invention, there is also provided a method for fabricating a semiconductor device having a fine pattern, which comprises the step of exposing a wafer substrate to light rays through a phase-shift photo mask comprising a phase-shift photo mask blank selected from the group consisting of a double-layered half tone phase-shift film in which the refractive index of the upper layer is smaller than that of the lower layer, a three-layered half tone phase-shift film in which the refractive index of the intermediate layer is smaller than those of the upper and lower layers or the refractive index of the upper layer is smaller than that of the intermediate layer, or a multi-layered half tone phase-shift film comprising at least 4 layers in which the refractive index of the upper most layer is smaller than that of the layer immediately below the upper most layer, on which a pattern to be transferred to the wafer substrate is formed.
The step of exposing a wafer to light rays can be performed using an F2 laser (xcexexp=157 nm), KrF excimer laser (xcexexp=248 nm), in addition to the ArF excimer laser (xcexexp=193 nm) already described above.